1. Field of the Invention
The invention relates to a frequency synthesizer, especially for a time base generator of a level measuring device which works according to the radar principle, with at least one first output for outputting of a first frequency signal, with at least one second output for outputting of a second frequency signal, and with a reference oscillator for producing a reference frequency signal, the first frequency signal and the second frequency signal having a small difference frequency relative to one another, the first frequency signal being producible by interaction of the reference oscillator with a direct digital synthesizer. Furthermore, the invention also relates to a level measuring device which works according to the radar principle with a frequency synthesizer for producing a first frequency signal—sampling signal—and for producing a second frequency signal—transmission signal.
2. Description of Related Art
Frequency synthesizers of the above mentioned type have been known for some time, especially in the field of measurement engineering. Frequency synthesizers which have two frequency signals with only a small difference frequency relative to one another are often used as a time base generator for level measuring devices which work according to the radar principle. The frequency synthesizer described here is also suitable for other applications, but the technical requirements imposed on frequency synthesizers can be explained especially well using distance measurements based on radar.
Level measuring devices which work according to the radar principle emit electromagnetic waves which typically have frequencies in the MHz to GHz range into the observed section of space. These electromagnetic waves are reflected on the contents which are to be detected and travel back to the level measuring device where they are, in turn, detected. The transit time of the transmission signal is proportional to the distance of the level measuring device—more accurately of the transducer of the level measuring device—from the contents. As a result of the propagation velocity of the electromagnetic waves near the speed of light in the monitored section of space, the transit times of the electromagnetic transmission signal which can, optionally, be in the region of a few nanoseconds are very small even if distances in the range of a few dozen centimeters are to be detected.
Detection of these small transit times imposes very high demands on the measurement engineering being used, it making no difference whether the emitted electromagnetic waves are emitted via an antenna or whether radar measurement takes place using a wave guide (time domain reflectometry).
In order to be able to detect reflected periodic electromagnetic waves correctly using measurement engineering, a sampling rate which corresponds to more than twice the frequency of the transmission signal (Nyquist criterion) is necessary, using a sampling process which works in real time. For sampling of reflected microwaves with the associated short signal transmit times, this necessitates sampling frequencies which can be managed only with exceedingly high circuitry cost.
In order to be able to detect such high-speed processes even with less complex means, the prior art discloses, not detecting a single reflected transmission signal with a high sampling rate, but detecting a host of reflected transmission signals in succession with sampling which is slightly offset in time from sampling step to sampling step, its being assumed that nothing changes in the situation which is to be detected using measurement engineering during sampling of the many different transmission pulses so that, therefore, a steady state is observed during measurement. This sampling is also known as a “serial sampling” or as “slow motion sampling”.
To implement this serial sampling, frequency synthesizers are used which output a first frequency signal at their first output and a second frequency signal at their second output, the first frequency signal and the second frequency signal having only a small frequency difference relative to one another. The second frequency signal is—to remain in the example of radar distance measurement—the transmission signal of a level measuring device which is emitted and later reflected. The first frequency signal is used for sampling which is continuously shifted farther, or for fixing the instant of this sampling, i.e., sampling of the second frequency signal so that the first frequency signal has a frequency which is slightly shifted relative to the second frequency signal. If the first frequency signal has a frequency f1, and the second frequency signal has a frequency f2, the frequency f1 of the first frequency signal and the frequency f2 of the second frequency signal differ by the difference frequency Δf according toΔf=|f2−f1|(1)
The ratio of the frequency f2 of the second frequency signal to the difference frequency Δf is the factor by which the reflected transmission signal to be scanned is stretched, and thus, is compressed accordingly in the frequency range, by which ultimately processing of the received signal is possible in a lower frequency range than that of the transmission signal. In practice, the difference frequency Δf between the first frequency signal and the second frequency signal is often in the region of only a few 100 Hz, partially even only at a few hertz, and therefore, in the frequency range in which the detection of the transmission signal using measurement engineering is easily possible.
In methods for distance measurement based on determining the transit time with serial sampling, the accuracy of the determined distance is directly related to the accuracy with which the difference frequency Δf can be maintained so that high measurement accuracy can be achieved here only by high stability at the difference frequency.
Known frequency synthesizers generally work with phase locked loops with which certain phases and frequency shifts that can be set, these circuits being comparatively complex since they require, for example, tunable oscillators (for example, VCO). Moreover these frequency synthesizers have a comparatively long transient recovery time in order to deliver a stable difference frequency or to change to another difference frequency.
Fundamentally, implementing a frequency synthesizer using direct digital synthesizers is also known; the latter are conventionally available as integrated semiconductor circuits and can produce a frequency-adjustable and phase-adjustable output signal which can be varied within wide limits and at high speed. Approaches are known in which the first frequency signal and the second frequency signal are each generated by separate direct digital synthesizers.
A direct digital synthesizer is a digital circuitry component which, on the output side, conventionally has a digital/analog converter and which makes it possible to produce a frequency-adjustable and phase-adjustable output signal, and the direct digital synthesizer itself can be clocked with a certain frequency signal. Essentially direct digital synthesizers have a so-called phase accumulator which is nothing more than a clocked counter which activates a phase storage. In the phase storage, the amplitude values of the frequency signal which belong to the respective phase are filed; therefore, for example, the amplitude values of a sinusoidal oscillation are stored. Due to clocked counting-up of the phase in the phase accumulator, the phase storage delivers the amplitude of the frequency signal which corresponds to the phase in digital form to a digital-analog converter which converts this value into an analog voltage signal. Often, the phase accumulators are controllable by it being possible to stipulate to them how many filed phase values are to be skipped for each increment. This property can then be parameterized by way of stipulating a shift word, the phase accumulator then fundamentally working as a variable modulus counter.
Aside from the fact that the approach to implementing a frequency synthesizer using two direct digital synthesizers is comparatively costly, it also has technical disadvantages which are related to the spectral interference which is conventionally produced by direct digital synthesizers.